The present invention relates to a system of cooling an assembly of nuclear fuel elements in a nuclear reactor, and in particular to a high temperature gas-cooled reactor (HTGR) having columns of stackable fuel elements with coolant passageways therein.
Nuclear reactors require effective heat transfer to utilize generated heat while maintaining acceptable limits on stresses and temperature levels within the core. Excessively uneven temperature distributions and higher local temperatures coupled with the given fast neutron fluence gradients can cause unacceptable stresses and fatigue in the fuel elements, which must be replaced more frequently as a consequence. Streams of relatively hot coolant, or "hot steams", exiting the core can stress surrounding materials such as those in the shielding and inlet ducts, requiring more frequent replacement and repair. Hot spots within the core can generate higher release of radioactive fission products, which, in turn, may cause higher circulating activity and plated-out activity on the surrounding metal parts. The radioactive metal requires additional protective measures in order for repairs to be effected. The higher core emissions also require greater shielding in general.
Uneven temperature distributions and high local temperatures can result from a number of causes which may vary according to reactor design. In some HTGRs the fuel is replenished through patch loading. A "patch" is a cluster of columns; for example, a central hexagonal column taken together with six adjacent columns constitutes a seven column patch. In patch-loaded HTGRs the periphery of the patch tends to be hotter than the center. This is due, in part, to the fact that adjacent patches may vary greatly in age since patches tend to be reloaded cyclically over a period of years. The power generation differential between adjacent patches of very different ages causes the portions of the columns near the boundary of the two patches to be hotter than the portions of the columns away from the boundary. Furthermore, when control rods are inserted into the center column of a patch, the power flux is driven toward the periphery of the patch contributing further to the heat differential between the patch periphery and the inner regions of the patch. Unevenness can also be generated in the regions of columns near side reflectors of the core.
In HTGRs having columns of stackable fuel elements, coolant flows through vertical coolant holes in the fuel elements and columns. Coolant flowing through the hotter regions of a column heats more rapidly than coolant flowing through the cooler regions of the reactor. Hotter coolant is less effective at cooling. This means that as the coolant approaches the bottom of the core, the least effective coolant flows through the regions in greatest need of cooling. On the other hand, the cooler regions of the column are cooled relatively effectively by the cooler flow therethrough, thereby aggravating the high temperature gradients across blocks. Furthermore, the exit temperatures of the hotter coolant can be much greater than that of nearby coolant flow, thereby creating hot streams which can cause fatigue in materials surrounding the reactor core.
The problems caused by large temperature gradients and high localized temperatures can be dealt with in part by variable orificing of the coolant holes. However, this approach is impractical on a column basis and reduces the total coolant flow for a given core pressure drop so that the average temperature is raised and heat transfer is generally diminished.